Semiconductor device

ABSTRACT

Provided is a semiconductor device including a buffer region. Provided is a semiconductor device including: semiconductor substrate of a first conductivity type; a drift layer of the first conductivity type provided in the semiconductor substrate; and a buffer region of the first conductivity type provided in the drift layer, the buffer region having a plurality of peaks of a doping concentration, wherein the buffer region has: a first peak which has a predetermined doping concentration, and is provided the closest to a back surface of the semiconductor substrate among the plurality of peaks; and a high-concentration peak which has a higher doping concentration than the first peak, and is provided closer to an upper surface of the semiconductor substrate than the first peak is.

The contents of the following Japenese patent application(s) areincorporated herein by reference:

-   -   NO. 2017-006187 filed in JP on Jan. 17, 2017, and        -   PCT/JP2018/000886 filed on Jan. 15, 2018.

BACKGROUND 1. Technical Field

The present invention relates to a semiconductor device.

2. Related Art

Conventionally, a semiconductor device in which a field stop regionhaving a plurality of impurity concentration peaks is formed is known(for example, refer to Patent Documents 1 and 2).

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1] U.S. Patent Application Publication No. 2016/0172438

[Patent Document 2] U.S. Patent Application Publication No. 2008/0001257

SUMMARY

Since the impurity concentrations of the respective peaks sequentiallyfall from the back surface side of the semiconductor substrate in theconventional field stop region, however, the conventional field stoplayer may not satisfactorily serve as a field stop region that isdesigned to stop a depletion layer when the device characteristics areoptimized.

A first aspect of the present invention may provide a semiconductordevice including a semiconductor substrate of a first conductivity type;

a drift layer of the first conductivity type provided in thesemiconductor substrate; and

a buffer region of the first conductivity type provided in the driftlayer, the buffer region having a plurality of peaks of a dopingconcentration. The buffer region may have a first peak which has apredetermined doping concentration, and is provided the closest to aback surface of the semiconductor substrate among the plurality ofpeaks; and

a high-concentration peak which has a higher doping concentration thanthe first peak, and is provided closer to an upper surface of thesemiconductor substrate than the first peak is.

The buffer region may further have a low-concentration peak that has alower doping concentration than the high-concentration peak and that isprovided closer to the upper surface of the semiconductor substrate thanthe high-concentration peak is.

The high-concentration peak may be a second peak provided the secondclosest next to the first peak to the back surface of the semiconductorsubstrate among the plurality of peaks.

The second peak may have the highest concentration among the pluralityof peaks.

The second peak may be provided at a position between 1 μm and 12 μminclusive from the back surface of the semiconductor substrate.

A doping concentration of a doping concentration valley between thefirst peak and the second peak may be 10 times or more as high as asubstrate concentration of the semiconductor substrate.

A doping concentration ratio N₂/N₁ of a doping concentration N₂ of thesecond peak to a doping concentration N₁ of the first peak may be1<N₂/N₁≤100.

The first peak may be a peak having the second highest concentrationnext to the second peak among the plurality of peaks.

The semiconductor substrate may have a doping concentration distributionin which a plurality of valleys are provided between the plurality ofpeaks, and doping concentrations of the plurality of valleyssequentially fall from a back surface side of the semiconductorsubstrate toward an upper surface side.

A ratio (N₁/N_(V12)) of a doping concentration N₁ of the first peak to adoping concentration N_(V12) of a doping concentration valley betweenthe first peak and the second peak may be higher than a ratio (N₂/N₁) ofa doping concentration N₂ of the second peak to the doping concentrationN₁ of the first peak.

The semiconductor device may have a transistor section and a diodesection. The diode section may have a floating layer of a secondconductivity type that is different from the first conductivity type andthe floating layer is closer to the back surface of the semiconductorsubstrate than the first peak is.

The semiconductor device may include a collector region of the secondconductivity type at the back surface of the transistor section.

Also, an integrated concentration from the collector region to thesecond peak may be higher than half of a critical integratedconcentration of the semiconductor substrate.

The buffer region may further include a third peak provided the thirdclosest next to the second peak to the back surface of the semiconductorsubstrate among the plurality of peaks. A doping concentration N₃ of thethird peak may be lower than a doping concentration N_(V12) of a dopingconcentration valley between the first peak and the second peak.

A distance X₂−X₁ in a depth direction between the second peak and thefirst peak may be smaller than a depth X₁ from a back surface of thesemiconductor substrate to the first peak.

A depth X₀ from the back surface of the semiconductor substrate to aboundary position between the first peak and the collector region may besmaller than a distance X₁−X₀ in a depth direction between (i) the firstpeak and (ii) the boundary position between the first peak and thecollector region.

A depth X₀ from the back surface of the semiconductor substrate to aboundary position between the first peak and the collector region may belarger than a distance X₁−X₀ in a depth direction between (i) the firstpeak and (ii) the boundary position between the first peak and thecollector region.

A distance X_(b) from the back surface of the semiconductor substrate toa boundary position between the first peak and the floating layer may besmaller than a distance X₁−X_(b), between (i) the first peak and (ii)the boundary position between the first peak and the floating layer.

A second aspect of the present invention provides a semiconductor deviceincluding a semiconductor substrate of a first conductivity type;

a drift layer of the first conductivity type provided in thesemiconductor substrate; and

a buffer region of the first conductivity type provided in the driftlayer, the buffer region having a plurality of peaks of a dopingconcentration. The plurality of peaks may include n peaks P_(n) of adoping concentration distribution (n is an integer greater than or equalto 2). The semiconductor device may include a pair of a peak and avalley satisfying NP_(i)/NB_(i+1)≤10, where NP_(i) denotes a dopingconcentration of an i-th peak P_(i) of the peaks P_(n) from a backsurface of the semiconductor substrate and NB_(i) denotes a dopingconcentration of a valley B_(i) that is closer to the back surface ofthe semiconductor substrate than the i-th peak P_(i) is.

The plurality of peaks may have a plurality of the pairs of a peak and avalley satisfying NP_(i)/NB_(i+1)≤10.

An envelope connecting all of doping concentrations NB_(i) of valleysmay attenuate from a valley B₁ to a valley B_(n+1) exponentially or moremoderately than an exponential function.

An envelope connecting all of doping concentrations NB_(i) of valleysmay attenuate convexly toward a side at which a doping concentration issmaller.

The summary clause does not necessarily describe all necessary featuresof the embodiments of the present invention. The present invention mayalso be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view briefly showing a semiconductor device100 according to an embodiment.

FIG. 1B is a cross-sectional view briefly showing a semiconductor device100 according to an embodiment.

FIG. 2 shows one example of a doping concentration distribution of asemiconductor substrate 10 according to an embodiment.

FIG. 3 shows one example of a doping concentration distribution of asemiconductor substrate according to the comparative example 1.

FIG. 4A shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.

FIG. 4B shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.

FIG. 4C shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.

FIG. 5 shows one example of a configuration of a semiconductor device100 having a floating layer 66.

FIG. 6A shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66.

FIG. 6B shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66.

FIG. 6C shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66.

FIG. 6D shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66.

FIG. 7 shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, (some) embodiments) of the present invention will bedescribed. The embodiment(s) do(es) not limit the invention according tothe claims, and all the combinations of the features described in theembodiment(s) are not necessarily essential to means provided by aspectsof the invention.

In the present specification, one side in a direction parallel to adepth direction of a semiconductor substrate is referred to as “upper”,and the other side is referred to as “lower”. One of two principalsurfaces of a substrate, a layer or some other member is referred to asan upper surface, and the other surface is referred to as a lowersurface. The “upper”, “lower”, “front”, and “back” directions are notlimited to the gravitational direction or the direction of attachment toa substrate or the like at the time of mounting of a semiconductordevice.

Although in each embodiment shown, the first conductivity type isN-type, and the second conductivity type is P-type, the firstconductivity type may be P-type, and the second conductivity type may beN-type. In this case, conductivity types of substrates, layers, regionsand the like in each embodiment have opposite polarities, respectively.

In the present specification, it is meant that the electrons or holesare majority carriers in the layers or regions specified with N or P,respectively. Also, ‘+’ and ‘−’ attached on ‘N’ and ‘P’ respectivelymean that the higher doping concentration and the lower dopingconcentration than the layer or region to which it is not attached.

FIG. 1A is a cross-sectional view briefly showing a semiconductor device100 according to an embodiment. The semiconductor device 100 is avertical semiconductor device in which an electrode is formed on anupper surface and a back surface of a semiconductor substrate 10 andcurrent flows in the thickness direction of the semiconductor substrate10. In the present example, a freewheeling diode (FWD) is illustrated asone example of the semiconductor device 100. The semiconductor device100 includes the semiconductor substrate 10, an upper-surface-sideelectrode 102 and a back-surface-side electrode 104.

The upper-surface-side electrode 102 is provided closer to the uppersurface of the semiconductor substrate 10. The upper-surface-sideelectrode 102 of the present example has a planar shape. However, theupper-surface-side electrode 102 of another example may have a trenchshape. If the semiconductor device 100 is the FWD, theupper-surface-side electrode 102 is an anode electrode.

The back-surface-side electrode 104 is provided at the back surface sideof the semiconductor substrate 10. If the semiconductor device is theFWD, the back-surface-side electrode 104 is a cathode electrode.

The semiconductor substrate 10 is formed of semiconductor material suchas silicon or a chemical compound semiconductor. The semiconductorsubstrate 10 is doped with impurities of a given concentration. Thesemiconductor substrate 10 has a first conductivity type. For example,the semiconductor substrate 10 is N−-type. In the present example, thefirst conductivity type is described as N-type while the secondconductivity type as P-type. However, the first conductivity type may beP-type while the second conductivity type may be N-type. Thesemiconductor substrate 10 has an upper-surface-side region 12, a driftregion 14, a buffer region 20, and a cathode region 21. The bufferregion 20 may function as a field stop layer that suppresses extensionof the depletion layer.

The drift region 14 has the first conductivity type. That is, the driftregion 14 of the present example has the same conductivity type as thesemiconductor substrate 10. The drift region 14 of the present exampleis of N−-type.

The upper-surface-side region 12 is arranged at the upper surface sideof the semiconductor substrate 10 and is doped with impurities of aconductivity type different from that of the drift region 14. In thepresent example, the upper-surface-side region 12 is P-type. If thesemiconductor device 100 is the FWD, the upper-surface-side region 12functions as an anode region.

The cathode region 21 is provided at the back surface side of thesemiconductor substrate 10. The cathode region 21 has the firstconductivity type. The cathode region 21 of the present example is ofN+-type. The cathode region 21 is one example of a back-surface-sideregion arranged between the buffer region 20 and the back-surface-sideelectrode 104.

The buffer region 20 is arranged at the back surface side of thesemiconductor substrate 10. The buffer region 20 has the sameconductivity type as that of the drift region 14 and is doped withimpurities at a higher concentration than the drift region 14. In thepresent example, the buffer region 20 is N+-type. The buffer region 20has a plurality of peaks 40. The buffer region 20 of the present examplehas six peaks 40-1, 40-2, 40-3, 40-4, 40-5, and 40-6 of the dopingconcentration. However, the number of the peaks 40 that the bufferregion 20 has is not limited to this. The plurality of peaks 40 may beformed by implanting impurities from the back surface side of thesemiconductor substrate 10 with different acceleration energies. Also,by implanting protons with changing the implantation amount per unitarea, the doping concentration can be adjusted according to the positionof the protons. Here, the doping concentration means a dopingconcentration of dop ants electrically activated as of N-type or P-type.

Also, the buffer region 20 suppresses the depletion layer which extendsfrom an interface of the upper-surface-side region 12 and the driftregion 14, from reaching the back-surface-side region of thesemiconductor substrate 10. The buffer region 20 is formed by implantingprotons from the back surface side of the semiconductor substrate 10 andperforming thermal treatment. The peaks 40 are formed by implantingsmall-mass impurities such as hydrogen ions such as protons, from theback surface side of the semiconductor substrate 10. By usinglightweight impurities such as protons, the positions of the peaks 40can be controlled with a high accuracy. The impurities are preferablysubstance having a smaller mass than phosphorus and selenium. Forexample, in a region into which protons are implanted, ahigh-concentration N+ layer is formed by hydrogen-induced donors inwhich the complex defects due to the implanted hydrogens and vacanciesand the like of the semiconductor substrate 10 become donors.

FIG. 1B is a cross-sectional view briefly showing a semiconductor device100 according to an embodiment. The semiconductor device 100 of thepresent example has a transistor section 50 and a diode section 70.

The transistor section 50 and the diode section 70 are provided beingadjacent to each other. The transistor section 50 of the present examplehas an insulated gate bipolar transistor (IGBT: Insulated Gate BipolarTransistor). The same configuration as the configuration illustrated inFIG. 1A is illustrated with the same reference numeral, unless otherwisespecified. Note that, if the semiconductor device 100 is the IGBT, theupper-surface-side electrode 102 is an emitter electrode, and theback-surface-side electrode 104 is a collector electrode.

Note that, in the present example, an example of a Reverse-conductingIGBT including a diode section 70 is described, but the semiconductordevice may be an IGBT not including a diode section 70. The embodimentdescribed below has the similar effect even with the IGBT not includinga diode section 70.

The semiconductor substrate 10 of the present example has a P-typeupper-surface-side region 12 which functions as a base region, anN−-type drift region 14, a buffer region 20, a collector region 52, anda cathode region 64. The semiconductor substrate 10 of the presentexample has a gate trench 54, an emitter trench 56, and an N+-typeemitter region 58 at the upper surface side. In the longitudinaldirection of the gate trench 54 or the emitter trench 56, the emitterregion 58 and the P+-type contact region (which is not shown) arealternately provided. The contact region has a higher dopingconcentration than that of the upper-surface-side region 12.

In the transistor section 50, a plurality of gate trenches 54 and aplurality of emitter trenches 56 that reach the drift region 14 from theupper surface of the semiconductor substrate 10 are provided. Inside thegate trench 54, a gate electrode G is arranged to which a gate voltageis applied. Inside the emitter trench 56, an emitter electrode E isarranged which is electrically connected to an upper-surface-sideelectrode 102 which functions as an emitter electrode. A P+-typecollector region 52 is provided at the back surface side correspondingto the transistor section 50.

The collector region 52 is a back-surface-side region between the bufferregion 20 and the back-surface-side electrode 104. The collector region52 has a conductivity type different from that of the semiconductorsubstrate 10. The collector region 52 of the present example has aconductivity type of a P+-type.

The insulating film 68 is arranged on the semiconductor substrate 10.The insulating film 68 of the present example is arranged between thegate electrode G and the emitter electrode E, and the upper-surface-sideelectrode 102. Also, in a part of regions in the insulating film 68, athrough hole is formed to connect the emitter electrode E and theupper-surface-side electrode 102.

In the diode section 70, a P-type upper-surface-side region 12 whichfunctions as a base region, an N−-type drift region 14, a buffer region20, and an N+-type cathode region 64 are provided from the upper surfaceside of the semiconductor substrate 10. In the diode section 70, aplurality of emitter trenches 56 that reach the drift region 14 from theupper surface of the semiconductor substrate 10 are provided. Note that,the back-surface-side electrode 104 is in contact with the collectorregion 52 and the cathode region 64 at the back surface of thesemiconductor substrate 10.

FIG. 2 shows one example of a doping concentration distribution of thebuffer region 20, among doping concentration distributions of asemiconductor substrate 10 according to FIG. 1A or FIG. 1B of anembodiment. The horizontal axis indicates the depth from the backsurface of the semiconductor substrate 10, and the vertical axisindicates the doping concentration. Here, illustration of the cathoderegion 64 which is the back surface outermost layer is omitted.

The doping concentration distribution of the present example includessix peaks at which the doping concentration reaches the local maximum,that is, the doping concentration locally turns from an increase to adecrease (peaks 40-1-40-6). Six peaks 40-1-40-6 are provided in thisorder from the back surface side of the semiconductor substrate 10. Thepeak 40-1 is one example of the first peak. The peak 40-2 is one exampleof the second peak.

The peak 40-1 has a predetermined doping concentration, and is providedthe closest to a back surface of the semiconductor substrate 10 amongthe plurality of peaks 40. The peak 40-1 of the present example has alower concentration than that of the peak 40-2.

The peak 40-2 has a higher doping concentration than the dopingconcentration of the peak 40-1. The peak 40-2 is provided closer to theupper surface of the semiconductor substrate 10 than the peak 40-1 is.The peak 40-2 is one example of the high-concentration peak P_(H). Thepeak 40-2 may be the highest concentration peak among the plurality ofpeaks 40.

The high-concentration peak P_(H) refers to the highest concentrationpeak among the plurality of peaks 40. The high-concentration peak P_(H)is provided closer to the upper surface of the semiconductor substrate10 than the peak 40-1 is, among the plurality of peaks 40. Thehigh-concentration peak P_(H) of the present example is provided thesecond closest next to the peak 40-1 from the back surface side of thesemiconductor substrate 10 among the plurality of peaks 40. That is, inthe present example, the second peak is the high-concentration peakP_(H).

The peak 40-3 to the peak 40-6 are provided closer to the upper surfacethan the peak 40-2 is. The peak 40-3 to the peak 40-6 have lower dopingconcentrations than the doping concentration of the high-concentrationpeak P_(H). The peak 40-3 and the peak 40-4 are provided closer to theupper surface of the semiconductor substrate 10 than thehigh-concentration peak P_(H) is. The peak 40-3 to the peak 40-6 are oneexample of the low-concentration peak.

That is, in the semiconductor substrate 10 of the present example, thedoping concentration of the peak 40-2 is higher than the dopingconcentration of the peak 40-1 and the doping concentrations of the peak40-3 to the peak 40-6. Accordingly, the semiconductor substrate 10 hasan upwardly-convex doping concentration distribution. In the presentexample, the case where the doping concentration of the peak 40-2 is thehigh-concentration peak P_(H) has been described. However, as long asthe semiconductor substrate 10 has an upwardly-convex dopingconcentration distribution, the peak 40 which is provided closer to theupper surface than the peak 40-2 is may be the high-concentration peakP_(H). Note that, in the present example, the doping concentration ofthe peak 40-6 is higher than the doping concentration of the peak 40-5.

In the present example, the peak 40-1 has the second highestconcentration next to the peak 40-2, which is the high-concentrationpeak P_(H). That is, the peak 40-1 has a higher doping concentrationthan those of the peak 40-3 to the peak 40-6. This enables the peak 40-1to surely stop the extension of the depletion layer from the uppersurface side of the semiconductor substrate 10. This enhances thebreakdown voltage of the semiconductor device 100.

Here, from the perspective of the manufacturing process, thesemiconductor substrate 10 may preferably have a doping concentrationdistribution in which the plurality of peaks 40 are arranged in andupwardly-convex manner. For example, in the step of forming the peaks40, when implanting ions into a region at a shallow position at the backsurface side of the semiconductor substrate 10, the acceleration energyat the time of implanting is lowered when compared with the case whenimplanting ions into a region at a deep position. If the accelerationenergy at the time of implanting is low, the amount of ions that can beimplanted for a certain time period is small. For this reason, theprocessing time of the step of forming the high-concentration peak 40 ina region at a shallow position at the back surface side of thesemiconductor substrate 10 is prolonged, and the manufacturingefficiency of the semiconductor device 100 deteriorates in some cases.In the semiconductor device 100 of the present example, because theshallowest peak 40-1 has a lower concentration than that of the peak40-2, the processing time for forming the peaks 40 can be shortened.Accordingly, this improves the manufacturing efficiency of thesemiconductor device 100.

Also, also from the perspective of optimization of devicecharacteristics, the semiconductor substrate 10 may preferably have adoping concentration distribution in which the plurality of peaks 40 arearranged in and upwardly-convex manner. For example, if ahigh-concentration peak 40 is formed at a region close to the backsurface of the semiconductor substrate 10, this may affect the amount ofholes implanted from the back surface of the semiconductor substrate 10.Because the semiconductor device 100 of the present example suppressesthe doping concentration of the peak 40-1 close to the back surface ofthe semiconductor substrate 10, the influences on the devicecharacteristics by the variations occur in the process steps are small.On the other hand, by making a doping concentration of a region at adeep position from the back surface of the semiconductor substrate 10higher, the function as a field stop layer for suppressing the depletionlayer which extends from the upper surface side of the semiconductorsubstrate 10 can be maintained. Thereby, the semiconductor device 100can suppress the leakage current.

The acceleration energy at which hydrogen ions irradiate thesemiconductor substrate 10 may have such a value that the hydrogen ionsdo not pass (or penetrate) through the semiconductor substrate 10.Conventionally, an energy absorbent member is installed between anaccelerator for hydrogen ions and a semiconductor substrate, andirradiation is performed at high acceleration energy (for example, 10MeV or more) such that hydrogen ions irradiating the semiconductorsubstrate penetrates the semiconductor substrate. In this case, theposition at which the hydrogen ions stop inside the semiconductorsubstrate (that is, the depth of the hydrogen ions is adjusted byadjusting the thickness or the like of the energy absorbent member,which is configured to absorb the energy of the hydrogen ions.

In the hydrogen ion irradiation at such high acceleration energy, damageto the semiconductor substrate is too large, and a full width at halfmaximum of the hydrogen ions is several dozen μm or more, so that adistribution width of lattice defects with respect to a thickness of thesemiconductor substrate is also increased. In contrast, if hydrogen ionsirradiate the semiconductor substrate at such acceleration energy thathydrogen ions stop inside the semiconductor substrate without using anenergy absorbent member, hydrogen induced donors can be formed withoutgiving excessive damages (that is, disorder).

When the energy absorbent member is not used and hydrogen ions irradiatethe semiconductor substrate, the acceleration energy may be set asfollows. If the semiconductor substrate is silicon, the relationship ofthe following equation (1) may be satisfied.

y=−0.0047x ⁴+0.0528x ³−0.2211x ²+0.9923x+5.0474   (1)

Here, Rp denotes the range of hydrogen ions in the semiconductorsubstrate (that is, the position at which the concentration of hydrogenions peaks), E denotes the acceleration energy of hydrogen ions, xdenotes the log (Rp), which is the logarithmic of the range Rp ofhydrogen ions, y denotes the log (E), which is the logarithmic of theacceleration energy E of hydrogen ions.

Note that, the acceleration energy E of hydrogen ion irradiation may becalculated (calculated value E) from the average range Rp of desiredhydrogen ions using the above curve fitting equation. When hydrogen ionsare implanted into the silicon substrate with the calculated value E ofthe acceleration energy, the relationship between the actualacceleration energy E′ and the average range Rp′ (hydrogen ions peakposition) actually obtained by Secondary Ion Mass Spectrometry (SIMS)and the like may be considered as follows. When the actual accelerationenergy E′ is within a range of about ±10% of the calculated value E, thecalculated value E of the acceleration energy, the actual average rangeRp′ also falls within a range of about ±10% of the desired average rangeRp to be within tolerance. For this reason, influences of variations ofthe actual average range Rp′ from the desired average range Rp onelectrical characteristics of IGBTs, diodes and the like aresufficiently small to a negligible extent. Therefore, if the actualacceleration energy E′ is within the range of ±5% of the calculatedvalue E, the actual average range Rp′ can be determined to besubstantially equal to the average range Rp as set. Alternatively, thereis no problem if the actual average range Rp′ falls within ±10% of theaverage range Rp that is assigned into the above equation (1) in orderto obtain the actual acceleration energy E′.

In the actual accelerator, because both the acceleration energy E andthe average range Rp can fall within the above range (±10%), thefollowing consideration is completely no problem: the actualacceleration energy E′ and the actual average range Rp′ conform with theabove-mentioned curve fitting equation that is expressed by a desiredaverage range Rp and the calculated value E. Furthermore, the range ofthe variation or error may be smaller than or equal to theabove-mentioned ±10% with respect to the average range Rp, andpreferably may fall within ±5% thereof. If the above mentioned conditionis satisfied, it can be considered that Rp′ conforms satisfactorily tothe above equation (1). By using the above equation (1), theacceleration energy E of hydrogen ions required to obtain the desiredrange Rp of hydrogen ions can be calculated.

Also, if the energy absorbent member is not used and hydrogen ions areimplanted with such an acceleration energy to such an extent thathydrogen ions remain in the semiconductor substrate, the full width athalf maximum ΔRp of hydrogen in the implantation direction may satisfythe following equation (2).

y=−0.00135x ⁶+0.01761x ⁵−0.07529x ⁴+0.08228x ³+0.21880x²+0.41061x−0.98152   (2)

Here, x denotes the log (Rp) which is the logarithmic of the range Rp ofhydrogen ions, and y denotes the log ΔRp which is the logarithmic of thefull width at half maximum (ΔRp).

Furthermore, when about 300° C. to 450° C. thermal treatment isperformed after implanting hydrogen ions, the implanted hydrogens arediffused inside the semiconductor substrate. For this reason, each peak40 of the buffer region 20 may have a doping concentration distributionwith a full width at half maximum with ΔRp′, the value which is 5 to 20times (preferably 7 to 10 times) as large as a full width at halfmaximum ΔRp satisfying the equation (2). That is, if the dopingconcentration of the buffer region 20 measured by the Spread ResistanceProfiling, the capacitance-voltage method (C-V method) or the likesatisfies ΔRp′, the implanted hydrogen ions can be considered to havebeen implanted at such an acceleration energy that ions irradiating thesemiconductor substrate do not pass (or penetrate) through thesemiconductor substrate 10.

Alternatively, more simply, if the full width at half maximum ΔD aboutthe doping concentration of each peak 40 is smaller than or equal to20%, preferably smaller than or equal to 10% of the depth D from theback surface of each peak 40, the implanted hydrogen ions may beconsidered to have been implanted at an acceleration energy of a valuesuch that hydrogen ions irradiating do not pass (or penetrate) thesemiconductor substrate 10.

As mentioned above, the semiconductor device 100 of the present examplehas the high-concentration peak P_(H) closer to the upper surface of thesemiconductor substrate 10 than the peak 40-1, the high-concentrationpeak P_(H) having a higher concentration than the peak 40-1, which isthe closest to the back surface of the semiconductor substrate 10. Thisenables the semiconductor device 100 to improve the manufacturingefficiency and to optimize the device characteristics. For example, thesemiconductor device 100 of the present example can optimize the ONvoltage Von and the tail current while the semiconductor device isturned off and the like.

FIG. 3 shows one example of a doping concentration distribution of asemiconductor substrate according to the comparative example 1. Thehorizontal axis indicates the depth from the back surface of thesemiconductor substrate, and the vertical axis indicates the dopingconcentration.

The doping concentration distribution of the present example includessix peaks 540-1-540-6. Six peaks 540-1-540-6 are provided in this orderfrom the back surface side of the semiconductor substrate 10. Thesemiconductor device of the present example has a doping concentrationdistribution in which the peaks 540 sequentially fall from the backsurface side of the semiconductor substrate. The peak 540-1 has a higherdoping concentration than the doping concentration of the peak 540-2.Then, the peak 540-1 is the high-concentration peak P_(H).

Here, when ion-implanting protons into a part at a shallow positionthrough the back surface of the semiconductor substrate, it is necessaryto suppress the acceleration energy to a low level. In this case,because the amount of ions that can be ion-implanted for a certain timeperiod decreases, the manufacturing efficiency may worsen with thedoping concentration distribution in which a doping concentration of apart at a shallow position through the back surface side of thesemiconductor substrate is high.

Also, the amount of holes implanted from the back surface of thesemiconductor substrate is set in order to optimize the characteristicsof the semiconductor device. Furthermore, the protons at a shallowposition close to the back surface boron at the back surface of thesemiconductor substrate may greatly affect the amount of holes implantedfrom the back surface. For this reason, it is necessary to appropriatelycontrol the dosage of protons. If the proton dosage closest to the backsurface is reduced in order to optimize the characteristics of thesemiconductor device, in the doping concentration distribution in whichthe peak concentrations sequentially fall, the total dosage of theentire proton layer becomes small. Because of this, peaks 40 may notserve as the buffer region for stopping the depletion layer. Also, ifthe total dosage of protons is small, dangling bonds of silicon atoms ina crystal defect portion generated by ion implantation cannot beterminally bonded with hydrogen, and the leakage current may increase.

FIG. 4A shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.The doping concentration distribution of the present example shows thedoping concentration distribution in the depth direction of thesemiconductor substrate 10 in the transistor section 50. For thisreason, a doping concentration C_(N) of the collector region 52 is shownat the back surface side of the semiconductor substrate 10.

The doping concentration distribution of the present example includesseven peaks 40-1-40-7. The seven peaks 40-1-40-7 are provided in thisorder from the back surface side of the semiconductor substrate 10. Thesubstrate concentration No refers to the doping concentration of thesemiconductor substrate 10. The doping concentration N₂ of the peak 40-2is higher than the doping concentration N₁ of the peak 40-1. Also, thepeak 40-3 to the peak 40-6 have a doping concentration distribution inwhich doping concentrations of the peak 40-3 to the peak 40-6sequentially fall from the back surface side toward the upper surfaceside of the semiconductor substrate 10. However, the dopingconcentration N₇ of the peak 40-7 of the present example is higher thanthe doping concentration N₆ of the peak 40-6.

The peak 40-1 is provided at the depth X₁ from the back surface of thesemiconductor substrate 10. The peak 40-2 is provided at the depth X₂from the back surface of the semiconductor substrate 10. That is, X₁<X₂holds. The depths of the plurality of peaks 40 may be appropriatelychanged according to the structure of the semiconductor device 100 andthe like. In one example, the depth X₂ of the peak 40-2 is between 1 μmand 12 μm inclusive. Also, the depth X₂ of the peak 40-2 may be between2 μm and 5 μm inclusive.

The distance X₂−X₁ in the depth direction between the peak 40-2 and thepeak 40-1 may be smaller than the depth X₁ from the back surface of thesemiconductor substrate 10, of the peak 40-1. In this case, X₂−X₁<X₁holds. This makes it difficult to change the speed at which thedepletion layer extends, and can suppress the vibration.

Also, the depth X₀ from the back surface of the semiconductor substrate10, of the boundary position between the peak 40-1 and the collectorregion 52, may be smaller than the distance X₁−X₀ in the depth directionbetween the peak 40-1, and the boundary position between the peak 40-1and the collector region 52. In this case, X₀<X₁−X₀ holds. This canreduce variations in the ON voltage Von.

The semiconductor substrate 10 has a doping concentration N_(v) of avalley of a doping concentration distribution between the peaks of theplurality of peaks 40. In the present specification, the valley of thedoping concentration distribution refers to the part where the dopingconcentration distribution shows the local minimum value. Thesemiconductor substrate 10 of the present example has a plurality ofvalleys. For example, the doping concentration N_(V12) refers to thedoping concentration of the doping concentration valley between the peak40-1 and the peak 40-2. Also, the doping concentration N_(V23) refers tothe doping concentration of the doping concentration valley between thepeak 40-2 and the peak 40-3. The doping concentration N_(V12) of thepresent example is larger than the substrate concentration No of thesemiconductor substrate 10. In one example, the doping concentrationN_(V12) is 10 times or more as high as a substrate concentration N₀ ofthe semiconductor substrate 10. This suppresses the spreading of thedepletion layer. Also, the doping concentration N_(V12) may be lowerthan the doping concentration N₃ of the peak 40-3.

The concentration ratio N₂/N₁ refers to the concentration ratio of thedoping concentration of the peak 40-2 to the doping concentration of thepeak 40-1. In one example, the concentration ratio N₂/N₁ is 1<N₂/N₁≤100.Also, the upper limit of the concentration ratio N₂/N₁ may be smallerthan or equal to 10, or may be smaller than or equal to 5. For example,if the lower limit of the concentration ratio N₂/N₁ is too low, defectsmay remain too much. On the other hand, if the concentration ratio N₂/N₁is too high, the ON voltage Von may fluctuate in relation to theimpurities forming the collector region 52 when the concentrationdeviates.

The doping concentrations N_(V) of the plurality of valleys sequentiallyfall from the back surface side toward the upper surface side of thesemiconductor substrate 10. That is, for the doping concentrationsN_(V12)−N_(V67) of the six valleys between each of the seven peaks40-1-40-7, N_(V12)>N_(V23), N_(V23)>N_(V34), N_(V34)>N_(V45),N_(V45)>N_(V56), and N_(V56)>N_(V67) are satisfied. Note that, in FIG.4A, in order to simplify the drawing, N_(V34), N_(V45), N_(V56), andN_(V67) are omitted.

Here, N₁/N_(V12) may be smaller than N₂/N₁. Thereby, when the depletionlayer exceeds X₂, the depletion layer can be suppressed between X₁ andX₂. Also, N₁/N_(V12) may be larger than or equal to N₂/N₁. Or,N₁/N_(V12) may be higher than N₂/N₁. This can facilitate implanting ofholes from the collector region 52 and suppress the deviation in the ONvoltage Von.

Here, the integrated concentration n_(p) from the position of the end atthe upper surface side of the collector region 52 on the back surface ofthe semiconductor substrate 10 to the peak 40-2 is higher than half ofthe critical integrated concentration n_(c) of the semiconductorsubstrate 10. Also, the integrated concentration n_(p) may be higherthan the critical integrated concentration n_(c). For example, thecritical integrated concentration n_(c) is indicated by the followingequation from the Poisson's equation.

n _(c)=ε_(s) ·E _(c) /q

Here, ε_(s) is the permittivity of the semiconductor, q is theelementary charge, and E_(c) is the dielectric breakdown electric fieldstrength of the semiconductor. For example, in the case of silicon,because E_(c) is 1.8E5-2.5E5 (V/cm), n_(c) is 1.2E12-1.6E12 (/cm²).

As mentioned above, in the semiconductor device 100 of the presentexample, by providing a peak 40-1 closer to the back surface of thesemiconductor substrate 10 than the high-concentration peak P_(H) is,defects on the back surface side of the semiconductor substrate 10 canbe terminally bonded. On the other hand, if the peak 40-1 is notprovided at the back surface side of the high-concentration peak P_(H),regions with many defects may remain. For example, the dopingconcentrations of the peak 40-1 and the peak 40-2 are preferably between1E15 cm⁻³ and 1E16 cm⁻³ inclusive. Note that, E means a power of 10, andfor example, 1E15 cm⁻³ means 1×10¹⁵ cm⁻³.

FIG. 4B shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.The doping concentration distribution of the present example isdifferent from the doping concentration distribution according to FIG.4A in that a doping concentration N₃ of the peak 40-3 is lower than adoping concentration N_(V12) of a doping concentration valley betweenthe peak 40-1 and the peak 40-2. That is, N₃<N_(V12) holds. Thereby, thebase doping concentrations of the peak 40-1 and the peak 40-2 are high,and the depletion layer which extends from the upper surface side of thesemiconductor substrate 10 can be easily stopped in the peak 40-1 andthe peak 40-2.

FIG. 4C shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.The doping concentration distribution of the present example isdifferent from the doping concentration distribution according to FIG.4A in that a distance X₀ of a boundary position between the peak 40-1and the collector region 52 from the back surface of the semiconductorsubstrate 10 is larger than a distance X₁−X₀ in a depth directionbetween the peak 40-1, and a boundary position between the peak 40-1 andthe collector region 52. That is, X₀>X₁−X₀ holds. In this manner, bydeepening the region in which the collector region 52 is formed,implantation of holes can be suppressed.

FIG. 5 shows one example of a configuration of a semiconductor device100 having a floating layer 66. The semiconductor device 100 of thepresent example is different from the semiconductor device 100 in FIG.1B in that it has a floating layer 66 in the diode section 70.

The floating layer 66 is provided at the cathode region 64 side of thediode section 70. The floating layer 66 of the present example isprovided closer to the back surface of the semiconductor substrate 10than the peak 40-1 is. Here, the position of the floating layer 66 maybe determined with a peak F_(P) at which the doping concentration of thefloating layer 66 becomes maximum as a reference. The floating layer 66has a conductivity type different from those of the plurality of peaks40. The conductivity type of the floating layer 66 of the presentexample is P+-type. The floating layer 66 is provided so as not tooverlap with the plurality of peaks 40. In this manner, by shifting thefloating layer 66 and the peaks of the buffer region 20, the influenceson the floating layer 66 by the buffer region 20 is reduced. Thereby,even if variations occur in the doping concentration distribution due tothe influences of the process, the influences on the devicecharacteristics is reduced.

FIG. 6A shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66. The dopingconcentration distribution of the present example shows the dopingconcentration distribution in the depth direction of the semiconductorsubstrate 10 in the diode section 70. For this reason, the dopingconcentration C_(N) of the collector region 52 and the dopingconcentration F_(P) of the floating layer 66 are shown on the backsurface side of the semiconductor substrate 10.

The doping concentration of the floating layer 66 is preferably higherthan the doping concentration of the peak 40-1. By making the dopingconcentration of the floating layer 66 high, the influences on the ONvoltage Von of the semiconductor device 100 by the plurality of peaks 40can be reduced. Then, the doping concentration of the floating layer 66may be higher than that of the high-concentration peak P_(H). This canfurther reduce the influences on the device characteristics of thesemiconductor device 100 by the plurality of peaks 40. Furthermore, thedoping concentration of the floating layer 66 may be higher than thedoping concentration of the peak 40-2.

Also, helium for lifetime control may be implanted at the back surfaceside of the semiconductor substrate 10. In this case, helium at the backsurface side is preferably implanted avoiding the peak position of thedoping concentration distribution. In one example, the back surfacehelium is implanted between the floating layer 66 and the peak 40-1.Also, the back surface helium may be implanted between the peak 40-1 andthe peak 40-2. Furthermore, the back surface helium may be implantedcloser to the upper surface side of the semiconductor substrate 10 thanthe high-concentration peak P_(H). This can reduce the influences on thedevice characteristics of the semiconductor device 100 by implantinghelium.

FIG. 6B shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66. The dopingconcentration distribution of the present example is different from thedoping concentration distribution according to FIG. 6A in that a dopingconcentration N₃ of the peak 40-3 is lower than a doping concentrationN_(V12) of a doping concentration valley between the peak 40-1 and thepeak 40-2. That is, N₃<N_(V12) holds. Thereby, the base dopingconcentrations of the peak 40-1 and the peak 40-2 are high, and thedepletion layer which extends from the upper surface side of thesemiconductor substrate 10 can be easily stopped in the peak 40-1 andthe peak 40-2.

FIG. 6C shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66. The dopingconcentration distribution of the present example is different from thedoping concentration distribution according to FIG. 6A in that adistance X₀ of a boundary position between the peak 40-1 and thecollector region 52 from the back surface of the semiconductor substrate10 is larger than a distance X₁−X₀ between the peak 40-1, and a boundaryposition between the peak 40-1 and the collector region 52. That is,X₀>X₁−X₀ holds. In this manner, by deepening the region in which thecollector region 52 is formed, implantation of holes can be suppressed.

FIG. 6D shows one example of a doping concentration distribution of asemiconductor substrate 10 having a floating layer 66. The dopingconcentration distribution of the present example is different from thedoping concentration distribution according to FIG. 6A in that adistance X_(b) of a boundary position between the peak 40-1 and thefloating layer 66 from the back surface of the semiconductor substrate10 is smaller than a distance X₁−X_(b) between the peak 40-1, and aboundary position between the peak 40-1 and the floating layer 66. Thatis, X_(b)<X₁−X_(b) holds. This can easily stop extension of thedepletion layer from the upper surface side of the semiconductorsubstrate 10 in the peak 40-1.

FIG. 7 shows one example of a more detailed doping concentrationdistribution of a semiconductor substrate 10 according to an embodiment.The doping concentration distribution of the present example shows thedoping concentration distribution in the depth direction of thesemiconductor substrate 10 in the transistor section 50. For thisreason, the doping concentration of the collector region 52 is shown atthe back surface side of the semiconductor substrate 10.

The buffer region 20 of the present example has n peaks P₁−P_(n) (n isan integer greater than or equal to 2) in the doping concentrationdistribution. Among n peaks P₁−P_(n), a doping concentration of the i-thpeak P_(i) from the back surface side of the semiconductor substrate 10is a doping concentration NP_(i). In the present example, the case forn=7 is shown. However, the present invention is not limited to thiscase. Note that, i is an integer greater than or equal to 1.

Also, the buffer region 20 has n+1 valleys B₁−B₊₁ in the dopingconcentration distribution. The valleys B₁−B_(n) are provided closer tothe back surface of the semiconductor substrate 10 than n peaks P₁−P_(n)are. Also, B_(n+1) is provided closer to the upper surface of thesemiconductor substrate 10 than the nth peak P_(n) is. Among the n+1valleys B₁−B₊₁, a doping concentration of the i-th valley B_(i) from aback surface side of the semiconductor substrate 10 is a dopingconcentration NB_(i). The doping concentration NB_(n+1) of the n+thvalley B₊₁ may be the same as the doping concentration of the driftregion 14.

Here, the buffer region 20 of the present example has a pair of a peakand a valley with NP_(i)/NB_(i+1)≤10. That is, the doping concentrationNP_(i) of the i-th peak P_(i) may be 10 times or less a dopingconcentration of the first valley B_(i+1) closer to the upper surface ofthe semiconductor substrate 10 than the peak P_(i) is. In this manner,by making the doping concentration ratio of a doping concentrationNP_(i) of the peak P_(i) to the doping concentration NB_(i+1) of thefirst valley B_(i+1) closer to the upper surface of the semiconductorsubstrate 10 than the peak P_(i) is 10 times or less, the spreading of aspace charge region while the semiconductor device is turned off and atthe time of reverse recovery can be made smooth. This achieves softswitching of the semiconductor device 100.

Also, the buffer region 20 may have a plurality of the pairs of a peakand a valley satisfying NP_(i)/NB_(i+1)≤10. For example, there are fouror more pairs of peaks P_(i) and valleys B_(i+1) withNP_(i)/NB_(i+1)≤10. Also, there may also be five or more pairs of peaksP_(i) and valleys B_(i+1) with NP_(i)/NB_(i−1)≤10. Note that, thepresent example has been described as NP_(i)/NB_(i+1)≤10, but thecondition may be NP_(i)/NB_(i+1)<10.

The valley B₁ is a valley which is the closest to the back surface sideof the semiconductor substrate 10. If the semiconductor device 100 isthe IGBT, the boundary between valley B₁ and the p-type collector region52 is a p-n junction. For this reason, the doping concentration of theshallowest valley B₁ is theoretically zero. Therefore, with respect tothe doping concentration of the valley B₁ at a boundary sandwiched bydifferent conductivity type regions in this manner, a donorconcentration (or acceptor concentration) at a position where a donorconcentration of an n-type region (the valley B₁ of the buffer region 20in the present example) and an acceptor concentration of a p-type region(the collector region 52 in the present example) are the sameconcentration, may be a doping concentration NB₁ of the valley B₁.

In the electric field strength distribution in the space charge region,the decrease in the electric field strength at a given depth position isdetermined based on the doping concentration NP_(i) of the peak P_(i)and the arrangement of the peak P_(i). On the other hand, the decreasein the average electric field strength in the entire depth direction ofthe semiconductor substrate 10 is determined based on the dopingconcentration NB_(i) of the valley B_(i) and the arrangement of thevalley B_(i) in addition to the doping concentration NP_(i) of the peakP_(i). That is, if the applied voltage is high, and thecollector-emitter voltage (or the cathode-anode voltage) is high, notonly the doping concentration of the peak but also the dopingconcentration of the valley and its distribution and arrangementcontribute to the extent of suppressing the spreading of the spacecharge region.

Therefore, an envelope E connecting all of doping concentrations NB_(i)of the valley B_(i) preferably attenuates moderately. For example, anenvelope E preferably attenuates from valley B₁ to valley B_(n+1)exponentially or more moderately than an exponential function. Also, theenvelope E may attenuate with a distribution rather than an exponentialfunction than the Gaussian function. The envelope E may show adistribution that is convex toward the side at which the dopingconcentration is low (the lower side of the graph), when the verticalaxis is a logarithmic scale and the horizontal axis is a linear scale.This can gradually increase the suppressing of the spreading of thespace charge region while the semiconductor device is turned off and ata time of reverse recovery toward the back surface of the semiconductorsubstrate 10 (that is, the collector region 52 or the cathode region64). Accordingly, the switching waveform of the semiconductor device 100becomes smooth, and soft switching of the semiconductor device 100 isachieved.

While the embodiments of the present invention have been described, thetechnical scope of the invention is not limited to the above describedembodiments. It is apparent to persons skilled in the art that variousalterations and improvements can be added to the above-describedembodiments. It is also apparent from the scope of the claims that theembodiments added with such alterations or improvements can be includedin the technical scope of the invention.

The operations, procedures, steps, and stages of each process performedby an apparatus, system, program, and method shown in the claims,embodiments, or diagrams can be performed in any order as long as theorder is not indicated explicitly by “prior to,” “before,” or the likeand as long as the output from a previous process is not used in a laterprocess. Even if the process flow is described using phrases such as“first” or “next” in the claims, embodiments, or diagrams, it does notnecessarily mean that the process must be performed in this order.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor substrate of a first conductivity type; a drift layer ofthe first conductivity type provided in the semiconductor substrate; anda buffer region of the first conductivity type provided in the driftlayer, the buffer region having a plurality of peaks of a dopingconcentration, wherein the buffer region has: a first peak which has apredetermined doping concentration, and is provided the closest to aback surface of the semiconductor substrate among the plurality ofpeaks; a high-concentration peak which has a higher doping concentrationthan the first peak, and is provided closer to an upper surface of thesemiconductor substrate than the first peak is; and a low-concentrationpeak that has a lower doping concentration than the high-concentrationpeak and that is provided closer to the upper surface of thesemiconductor substrate than the high-concentration peak is, wherein thehigh-concentration peak is a second peak provided the second closestnext to the first peak to the back surface of the semiconductorsubstrate among the plurality of peaks, a ratio (N₁/N_(V12)) of a dopingconcentration N₁ of the first peak to a doping concentration N_(V12) ofa doping concentration valley between the first peak and the second peakis higher than a ratio (N₂/N₁) of a doping concentration N₂ of thesecond peak to the doping concentration N₁ of the first peak.
 2. Thesemiconductor device according to claim 1, wherein the second peak hasthe highest concentration among the plurality of peaks.
 3. Thesemiconductor device according to claim 1, wherein the second peak isprovided at a position between 1 μm and 12 μm inclusive from the backsurface of the semiconductor substrate.
 4. The semiconductor deviceaccording to claim 1, wherein a doping concentration of a dopingconcentration valley between the first peak and the second peak is 10times or more as high as a substrate concentration of the semiconductorsubstrate.
 5. The semiconductor device according to claim 1, wherein adoping concentration ratio N₂/N₁ of a doping concentration N₂ of thesecond peak to a doping concentration N₁ of the first peak is1<N₂/N₁≤100.
 6. The semiconductor device according to claim 1, whereinthe first peak is a peak having the second highest concentration next tothe second peak among the plurality of peaks.
 7. The semiconductordevice according to claim 1, wherein the semiconductor substrate has adoping concentration distribution in which a plurality of valleys areprovided between the plurality of peaks, and doping concentrations ofthe plurality of valleys sequentially fall from a back surface side ofthe semiconductor substrate toward an upper surface side.
 8. Thesemiconductor device according to claim 1, comprising a transistorsection and a diode section, wherein the diode section has a floatinglayer of a second conductivity type that is different from the firstconductivity type and the floating layer is closer to the back surfaceof the semiconductor substrate than the first peak is.
 9. Thesemiconductor device according to claim 8, comprising a collector regionof the second conductivity type at the back surface of the transistorsection, wherein an integrated concentration from the collector regionto the second peak is higher than half of a critical integratedconcentration of the semiconductor substrate.
 10. The semiconductordevice according to claim 1, wherein the buffer region further has athird peak provided the third closest next to the second peak to theback surface of the semiconductor substrate among the plurality ofpeaks, wherein a doping concentration N₃ of the third peak is lower thana doping concentration N_(V12) of a doping concentration valley betweenthe first peak and the second peak.
 11. The semiconductor deviceaccording to claim 1, wherein a distance X₂−X₁ in a depth directionbetween the second peak and the first peak is smaller than a depth X₁from the back surface of the semiconductor substrate to the first peak.12. The semiconductor device according to claim 9, wherein a depth X₀from the back surface of the semiconductor substrate to a boundaryposition between the first peak and the collector region is smaller thana distance X₁−X₀ in a depth direction between (i) the first peak and(ii) the boundary position between the first peak and the collectorregion.
 13. The semiconductor device according to claim 9, wherein adepth X₀ from the back surface of the semiconductor substrate to aboundary position between the first peak and the collector region islarger than a distance X₁−X₀ in a depth direction between (i) the firstpeak and (ii) the boundary position between the first peak and thecollector region.
 14. The semiconductor device according to claim 8,wherein a distance X_(b) from the back surface of the semiconductorsubstrate to a boundary position between the first peak and the floatinglayer is smaller than a distance X₁−X_(b), between (i) the first peakand (ii) the boundary position between the first peak and the floatinglayer.
 15. A semiconductor device comprising: a semiconductor substrateof a first conductivity type; a drift layer of the first conductivitytype provided in the semiconductor substrate; and a buffer region of thefirst conductivity type provided in the drift layer, the buffer regionhaving a plurality of peaks of a doping concentration, wherein theplurality of peaks include n peaks P_(n) of a doping concentrationdistribution (n is an integer greater than or equal to 5), thesemiconductor device comprises five or more pairs of a peak and a valleysatisfying NP_(i)/NB_(i+1)≤10, where NP_(i) denotes a dopingconcentration of an i-th peak P_(i) of the peaks P_(n) from a backsurface of the semiconductor substrate and NB_(i) denotes a dopingconcentration of a valley B_(i) that is closer to the back surface ofthe semiconductor substrate than the i-th peak P_(i) is.
 16. Thesemiconductor device according to claim 15, wherein an envelopeconnecting all of doping concentrations NB_(i) of valleys attenuatesfrom a valley B₁ to a valley B_(n+1) exponentially or more moderatelythan an exponential function.
 17. The semiconductor device according toclaim 15, wherein an envelope connecting all of doping concentrationsNB_(i) of valleys attenuates convexly toward a side at which a dopingconcentration is smaller.